(1) Field of the Invention
The present invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of overcoming the separation problem between phosphosilicate glass and tetraethoxysilane (TEOS) oxide in the fabrication of integrated circuits.
(2) Description of the Prior Art
U.S. Pat. Nos. 4,872,947 to Wang and 5,219,774 to Vasche describe O.sub.3 -TEOS processes. U.S. Pat. No. 4,894,352 to Lane et al teaches the addition of NF.sub.3 to TEOS deposition to increase the deposition rate. U.S. Pat. No. 5,013,691 to Lory et al teaches a TEOS deposition process using NH.sub.3 or NF.sub.3 gas to inhibit deposition, then removes the gas from horizontal surfaces using high radio frequency power so that horizontal TEOS deposition is favored.
Borophosphosilicate glass (BPSG) or phosphosilicate glass (PSG) is often used as an interlevel dielectric material. The BPSG or PSG is deposited over the underlayers and then flowed, typically at a temperature of about 900.degree. C. for about 30 minutes in a N.sub.2 ambient. This planarizes the surface of the substrate. It is proposed that during the flow treatment, outgassing from the BPSG or PSG layer occurs. For example, boron, phosphorus, carbon, and hydrogen gases are emitted from a BPSG layer during flow treatment. Referring now to FIG. 1, there is shown a semiconductor substrate 10 on which a layer of BPSG 12 has been deposited. A metal layer 14 is deposited and patterned. Impurities R form on the metal and BPSG surfaces, possibly due to outgassing caused by the BPSG flow process. These impurities can be, for example, BPO.sub.4, PO.sub.3, B.sub.2 O.sub.5, etc. Because of the presence of the impurities R, the initial deposition of TEOS 18 is not stable, especially on the BPSG/TEOS interface. The actual TEOS deposition mechanism is not known. However, it is known that TEOS is hydrophobic as a gas molecule and prefers to deposit on hydrophobic surfaces, especially at atmospheric pressure. BPSG and PSG surfaces are hydrophillic; that is, there is more OH bonding. Therefore, it is reasonable to assume that TEOS molecules would be repelled from the BPSG or PSG surface during deposition. The hydrophobic property of TEOS is discussed by Sato et al in "Improvement of Gap-Filling Property of O.sub.3 -tetraethylorthosilicate (TEOS) Film by Ethanol Surface Treatment," Japan Journal of Applied Physics 32 (1993) Pt. 2 No. 1A/B pp. L110-L112.
An experiment was devised to simulate the separation problem. As shown in FIG. 2, a wafer was coated with BPSG which was flowed at 900.degree. C. for 30 minutes in a N.sub.2 ambient. A metal layer 14 of AlSiCu was sputter deposited over the BPSG layer 12 and patterned. The wafer was treated with isopropyl alcohol (IPA) to put impurities on the metal surface. The wafer was spin-coated with IPA, then the wafer was dried by baking in a 100.degree. C. oven. In general, organic groups such as R--OH are formed as impurities on the metal surface.
Next, an atmospheric pressure TEOS oxide deposition 16 was performed using an O.sub.3 /TEOS feed ratio of 40:1 and deposition temperature of 400.degree. C. to a film thickness of about 6000 Angstroms. During integrated circuit production, the film separation between the BPSG 12 and TEOS 16 layers would be found during reliability testing. In this experiment, the wafer was treated for 5 seconds in a HF containing stain solution. If the film interface is intrinsically weak, the interface will be attacked by the stain solution. The sample was examined by cross section under a scanning electron microscope (SEM). A separation was found between the BPSG 12 and the TEOS 16. It is desirable to find a method to avoid the separation problem between the BPSG and the TEOS layer.